In the manufacturing industry, iron materials have been gradually replaced by lightweight materials such as aluminum and the like. The necessity for lightweight materials has culminated in the development of an aluminum alloy that can form a structure withstanding the stress corresponding to that of a structure formed of iron materials. Such an aluminum alloy must be able to have corrosion resistance, be die-cast and be easily machined as well as have high yield strength and high elongation.
Historically, a cast aluminum alloy has been characterized by having low strength and ductility compared to a forged product having a composition similar to that of the cast aluminum alloy. The reason why the cast aluminum alloy has low strength and ductility is because the cast aluminum alloy has defects that are generally removed by machining the forged aluminum alloy. Such effects are classified into two types, that is, pores caused by contraction or gas storage and large breakable particles caused by the intermetallic phase formed by oxides or impurities trapped in a cast product. High-quality cast products result from developing casting technologies for minimizing the number and size of such defects or changing the composition of aluminum alloy.
The highest-quality cast aluminum alloy is aluminum-silicon-magnesium (Al—Si—Mg) alloy. The strength and ductility of an aluminum alloy can be generally improved by maintaining aluminum alloy clean or using high-purity components (reforming AlSiFe 5 by increasing the content of iron (Fe) and/or by adding beryllium (Be)). As a result, currently, the properties of aluminum cast products are approaching those of aluminum forged products having the same composition as the aluminum cast product.
However, with the development of industry, aluminum alloys having improved mechanical properties have been required, and thus aluminum alloys having high thermal conductivity for die casting have also been required.
Most commercially available aluminum alloys for die casting are complex alloys each including several alloys and impurity elements. Due to the elements included in the complex alloy, the variable concentration thereof and the interaction therebetween, systematic research into the effect of the elements on commercially available aluminum alloys is complicated and difficult.
Although it is difficult to explain the effect of each element on the mechanical properties of aluminum alloys, it is recognized by those skilled in the art that the properties of aluminum alloys are influenced by magnesium, manganese, iron, silicon and beryllium as follows.
Magnesium is generally used to improve the tensile strength of aluminum alloy. A binary alloy of Al—Mg has high strength, excellent corrosion resistance, excellent weldability and excellent surface finishability. However, when the content of magnesium is increased, the hardness and fatigue endurance of aluminum alloy can be improved, but the ductility of aluminum alloy may be decreased. The reason why the content of magnesium in aluminum alloy is limited is because magnesium is easily oxidized to form magnesium oxide (MgO) particles in the molten aluminum alloy. That is, spinel, which is a complicated aluminum magnesium oxide, is formed at high temperature (750° C. or more), and thus an inclusion is formed in the molten aluminum alloy and rapidly grows. Such an inclusion decreases the fluidity and elongation of the aluminum alloy.
Copper (Cu) may also be added to the aluminum alloy in order to increase the strength and thermal conductivity of the aluminum alloy. When the content of copper is increased, the hardness and thermal conductivity of the aluminum alloy are increased, but the strength and ductility thereof depend on whether or not copper (Cu) is present in a solid solution or exists in the form of spheroidal or uniformly-applied particles. Copper (Cu) decreases electrolytic potential and corrosion resistance. The aluminum alloy containing copper is greatly spotted and corroded when it is annealed, and may be interparticle-corroded or stress-corroded even when it is aged and cured.
Silicon (Si) is an important component for improving the fluidity of molten aluminum alloy during a die casting process. An Al—Si alloy has good high-temperature tear resistance, steadiness and weldability because it has low contractility and a narrow freezing point range. In an Al—Mg alloy, silicon (Si) increases ductility and extensibility without increasing strength. Further, in an Al—Cu—Si alloy, a combination of copper and silicon greatly increases hardness, but greatly decreases extensibility.
Iron (Fe) is generally added to a die casting aluminum alloy in order to prevent the aluminum alloy from becoming attached to the die and to easily detach the aluminum alloy from the die. However, the extensibility of the aluminum ally is decreased by the addition of iron (Fe). In order to solve the problem, manganese (Mn) is added to the aluminum alloy. However, when an excessive amount of manganese (Mn) is added, the mechanical strength of the aluminum alloy may be lowered.
A LED bulb, which has lately been developed and used, must have a body structure for radiating the heat emitted therefrom. However, currently, commercially-available die casting materials include ADC12 (LM2), ADC1 (LM6), B390 and DM3H. Here, ADC12 (LM2) has a thermal conductivity of 100 W/mk, ADC1 (LM6) has a thermal conductivity of 142 W/mk, B390 has a thermal conductivity of 134 W/mk, and DM3H has a thermal conductivity of 114 W/mk.
DM3H is an anodizable material, but has a low thermal conductivity of 114 W/mk. Further, ADC12 is a die casting material having good mass productivity, but has a low thermal conductivity of 100 W/mk.
Meanwhile, 6063 is a material having the highest thermal conductivity, is a magnesium (Mg) alloy, and is used as a heat sink. Although 6063 has a high thermal conductivity of 190˜200 W/mk, it can be die-cast because it easily breaks.
It is not easy to obtain a material which has high thermal conductivity and can also be die-cast. Therefore, it is difficult to obtain a material suitable for a product such as LED or the like which is manufactured by die casting and which must radiate heat.